(1) Field of the Invention
The present invention relates to the system for receiving and reproducing digital broadcasting. More specifically, it relates to methods for correcting clock frequency, receiving apparatuses, reproducing apparatuses, and programs.
(2) Description of the Related Art
Digital broadcasting has been rapidly growing as can be seen in commencement of the CS digital broadcasting and the BS digital broadcasting. The digital broadcasting is realized by such systems that comprise a broadcasting apparatus and a receiving/reproducing apparatus. The broadcasting apparatus generates a plurality of pieces of broadcasting data by encoding digital content such as videos and sounds, and then transmits the pieces of broadcasting data in an order. The receiving/reproducing apparatus receives and decodes the broadcasting data, and then reproduces the content.
The broadcasting apparatus and the receiving/reproducing apparatus each contain a standard clock. By clocking based on the standard clock, time for the broadcasting apparatus (hereinafter referred to as “broadcasting-side time”) and time for the receiving/reproducing apparatus (hereinafter referred to as “reproducing-side time”) are generated and possessed respectively. In the MPEG-2 standard, content is transmitted with attached information called a timestamp, indicating the time to reproduce the content. When encoding the content, the broadcasting apparatus attaches the timestamp to the content by referring to the broadcasting-side time. The receiving/reproducing apparatus reproduces the content when the attached timestamp and the reproducing-side time are identical.
However, since the standard clock contained in the receiving/reproducing apparatus is independent of the standard clock of the broadcasting apparatus, a discrepancy between the broadcasting-side time and the reproducing-side time occurs, in the cases where there is an error between the standard clock frequency of each apparatus. Such a discrepancy in the time could grow larger, if left uncorrected, to an extent that the content cannot be reproduced properly.
In order to correct the discrepancy, a standard clock correcting mechanism for correcting the standard clock of the receiving/reproducing apparatus is provided. In this mechanism, the broadcasting apparatus broadcasts broadcasting-side time information, which is an encoded broadcasting-side time, contained in at least one of the pieces of broadcasting data in addition to the content such as videos and sounds; the receiving/reproducing apparatus calculates the error between each standard clock frequency, based on the broadcasting-side time information contained in the received broadcasting data, and corrects the standard clock frequency of the receiving/reproducing apparatus. By such a standard clock correcting mechanism, the broadcasting-side time and the reproducing-side time can be set approximately same, and the reproduction of the content can be carried out properly. In the following paragraphs, the standard clock correcting mechanism is explained in reference to FIG. 1.
FIG. 1 shows a construction of a conventional receiving/reproducing apparatus.
First, the broadcasting data passes through a time information extracting unit 601 in a receiving/reproducing apparatus 606. If the broadcasting data contains the broadcasting-side time information, the time information extracting unit 601 transmits the broadcasting-side time information to broadcasting-side time information holding unit 602, and notifies a time information managing unit 603 that the time information is received by a signal indicating the reception. In response to the signal sent from the time information extracting unit 601, the time information managing unit 603 reads out receiving/reproducing-side time information at the time when the broadcasting-side time information is received from a receiving/reproducing-side time information holding unit 604. The time information managing unit 603 compares the broadcasting-side time information and the receiving/reproducing-side time information, calculates the difference between each standard clock frequency, and then notifies a variable frequency generating unit 605 an amount of correction. The variable frequency generating unit 605 corrects the frequency according to the notified amount of correction.
In recent years, portable terminals such as mobile phones and personal digital assistances (PDAs) have become explosively widespread, as higher performance and more reduction in both weight and price of such portable terminals have been realized with gathering pace. With the popularization of the digital broadcasting in the future, it is expected that such portable terminals will be provided with functions to receive digital broadcasting. In order to receive digital broadcasting with such portable terminals, such a system explained below is considered to be highly possible. In the system, an expansion card which is a small storage device such as secure digital memory cards and memory sticks provided with broadcast receiving functions is used as a receiving apparatus; a portable terminal is used as a reproducing apparatus. By connecting the expansion card and the portable terminal, a system to receive digital broadcasting with portable terminals can be realized.
Such small storage devices and the portable terminals are usually connected via bus. Therefore, it is desirable that the broadcasting data received through the receiving apparatus is transmitted to the reproducing apparatus via bus.
In the case of transmission via bus, however, the transmission of the broadcasting data becomes on hold if bus is occupied, while, when bus is idle, it can be transmitted to the reproducing apparatus immediately after the broadcasting data is received. This indicates that the length of the time for transmitting the broadcasting-side time information from the receiving apparatus to the reproducing apparatus is uncertain. In order to correct the standard clock frequency based on the broadcasting-side time information, not only a content of the broadcasting-side time information, but also information about timing of the reception of the broadcasting-side time information is necessary, and thus correction of the standard clock frequency cannot be carried out properly in the cases in which bus are used for transmission.